1. Field of the Invention
The present invention relates to an end-modified styrene-diene block copolymer for use in hot-melt adhesives having new microstructures. More particularly, the present invention relates to an end-modified styrene-diene block copolymer made by reacting an end of styrene-diene copolymer with a polyfunctional polysiloxane as a new coupling agent.
2. Description of the Related Art
According to known documents, a styrene-diene block copolymer for use in hot-melt adhesives can be obtained in the form of linear PS-PD-PS block copolymer or radial (PS-PI)nX or (PS-PI-PB)nX by copolymerizing alkenyl arene and diene using an alkyl lithium as an initiator and then reacting the obtained copolymer with a bifunctional, trifunctional or tetrafunctional coupling agent. In this case, PS is a polymer block of an alkenyl arene, PD is a polymer block of a diene, PI is a polymer block of an isoprene, PB is a polymer block of a butadiene, X is a residue of a polyfunctional coupling agent after the coupling reaction, and n is 3 or 4.
U.S. Pat. No. 5,194,500 discloses a styrene-isoprene block copolymer having a general structure of (PS-PI)3X made using a trifunctional coupling agent for use in hot melt adhesives to thereby ensure improved physical properties of the adhesives. U.S. Pat. Nos. 5,292,819 and 5,399,627 disclose advantages of hot-melt adhesives based on (PS-PI-PB)nX block copolymers. U.S. Pat. No. 5,627,234 discloses the use of the resulting mixture obtained by reacting an alkenyl arene compound and a diene compound with various types of coupling agents in the composition of adhesives. U.S. Pat. No. 5,668,208 discloses that (PS-PI)3(OR)nOH copolymer based hot-melt adhesives have superior adhesive property to conventional hot-melt adhesives based on (PS-PI)3X copolymer having three arms. However, there is still a need to improve adhesive strength of adhesives commonly used for industrial purposes. In particular, because most adhesive sites are polar, with regard to conventional styrene-diene block copolymer based adhesives, improvement of adhesive strength on polar surfaces is required.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an end-modified polymer with low melt viscosity and excellent adhesive strength by use of a polyfunctional polysiloxane as a coupling agent.
In accordance with the present invention, the above object and other objects can be accomplished by the provision of a radial styrene-diene block copolymer having a general formula 1, made by copolymerizing styrene and diene using an organic lithium initiator in the presence of an organic solvent which does not react with the organic lithium and then end-modifying the resulting copolymer using a polysiloxane compound as a coupling agent:
(PS-PD)3Sixe2x80x94CH2CH2xe2x80x94[SiO(CH3)2xe2x80x94]nSi(CH3)2Rxe2x80x83xe2x80x83Formula 1
wherein,
PS is polystyrene, PD is polydiene such as polyisoprene or polyisoprene-polybutadiene, R is hydroxy group or -(PD-PS), and n is an integer of 1 to 100.
Hereinafter, the present invention will be described in more detail.
A radial end-modified styrene-diene block copolymer having a general formula 1 is obtained by synthesizing styrenediene block copolymer by an anionic copolymerization using an organic lithium initiator in the presence of an organic solvent that does not react with the organic lithium, and then coupling the anionic active end of the resulting copolymer with a polyfunctional polysiloxane:
(PS-PD)3Sixe2x80x94CH2CH2xe2x80x94[SiO(CH3)2xe2x80x94]nSi(CH3)2Rxe2x80x83xe2x80x83Formula 1
wherein,
PS is polystyrene, PD is polydiene such as polyisoprene or polyisoprene-polybutadiene, R is hydroxy group or -(PD-PS), and n is an integer of 1 to 100.
A number average molecular weight of the polystyrene (PS) in the formula 1 is 5,000 to 20,000, and preferably 8,000 to 15,000. A number average molecular weight of the polydiene (PD) is 40,000 to 150,000, and preferably 50,000 to 130,000. The weight ratio of polyisoprene/polydiene is 50% or more by weight, preferably 90% or more by weight. A number average molecular weight of the end-modified polymer of the formula 1 is 200,000 to 500,000, and preferably 250,000 to 400,000. The total content of the polystyrene block is at least 10 to 30% by weight, preferably 17 to 25% by weight of the polymer of the present invention. The polymer of the present invention is a radial polymer having three to four polystyrene-polydiene diblocks. The end of the polymer that reacts with a polyfunctional polysiloxane coupling agent is isoprene or butadiene. If the polydiene is polyisoprene-polybutadiene, the weight ratio of polyisoprene/polydiene is 50% or more by weight, and preferably 90% or more by weight. The coupling agent which is used herein is a xcex1,xcfx89-bis(2-trichlorosilylethyl)polydimethylsiloxane and was prepared according to the method disclosed in Korean Patent No. 0332466. An anionic polymerization is used in the present invention. Added alkenyl arenes and dienes are subsequently polymerized using an organic lithium compound in the presence of a suitable organic solvent that does not react with anions. Hydrocarbon compounds are used as the organic solvents herein and are solvents that do not react with anions. They are selected from the group consisting of linear hydrocarbon compounds such as pentane, hexane, heptane and octane; branched derivatives thereof; cyclic hydrocarbon compounds such as cyclohexane and cycloheptane; aromatic hydrocarbon compounds such as benzene, toluene and xylene; linear or cyclic ether compounds such as dimethyl ether, diethyl ether, anisole and tetrahydrofuran. The organic lithium compound used in the anionic polymerization is n-butyl lithium, s-butyl lithium or t-butyl lithium. A temperature for the anionic polymerization is generally within the range of from 20xc2x0 C. to 100xc2x0 C., and a reaction pressure is generally within the range of from atmospheric pressure to 5 kgf/cm2. An end-modified polymer obtained by reaction of anionic active site of an end of polymer and polyfunctional coupling agent has a coupling efficiency that can be calculated as follows:
(Coupled polymer area)/(Total polymer area)xc3x97100
The coupling efficiency can be calculated as an area of polymer peak measured by gel permeation chromatography (GPC). The coupling number (CN) can also be calculated by GPC as follows:
(Coupled polymer peak molecular weight after coupling/Polymer peak molecular weight before coupling).
Typically, a satisfactory value of the coupling efficiency is within the range of from 60% to 95%, and preferably from 75% to 90%. If the coupling efficiency is less than 60%, physical properties of adhesives are undesirably lowered, while if it exceeds 95%, a melt viscosity is increased and thus workability is not easily obtained. It is also difficult to obtain coupling efficiency of more than 95%. The adhesive composition of the present invention comprises a plasticizer oil, a tackifier, an antioxidant and the like. The plasticizer oil to be used herein is a paraffin oil or a naphthenic oil. The tackifier is present in liquid phase at room temperature and includes C5 resin or C9 resin. The antioxidant is also used as an additive herein and includes a steric hindered phenolic compound with high molecular weight, phosphorus- or sulfur-containing phenol, and the like. For example, pentaerythritol-tetrakis-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate; 4,4xe2x80x2-methylenebis(4-methyl-6-t-butylphenol); di-n-octadecyl-3,5-di-t-butyl-4-hydroxybenzophophonate; and the like may be used.